Solid electrolyte ion transport membranes have significant potential for the separation of oxygen from gas streams containing oxygen. Of particular interest are mixed conductor materials that conduct both oxygen ions and electrons and hence can be operated in a pressure driven mode without the use of external electrodes.
In an ionic or mixed conducting membrane reactor, a solid electrolyte membrane that can conduct oxygen ions with infinite selectivity is disposed between an oxygen-containing feed stream and an oxygen-consuming, typically methane-containing, product or purge stream. The membrane elements have "oxygen selectivity," which means that oxygen ions are exclusively transported across the membrane without transport of other elements, and ions of other elements. Such membranes may also be used in gas purification applications as described in European Patent Application Publication No. 778,069 entitled "Reactive Purge for Solid Electrolyte Membrane Gas Separation," issued to Prasad et al.
Composite ceramic mixed conductor membranes comprised of multi-phase mixtures of an oxygen ion conductive material and an electronically-conductive material are known. Exemplary multi-phase ceramic compositions of this type are disclosed in U.S. Pat. No. 5,306,411 (Mazanec et al.) and U.S. Pat. No. 5,478,444 (Liu et al.). Such compositions are also taught by C. S. Chen et al. in Microstructural Development, Electrical Properties and Oxygen Permeation of Zirconia-Palladium Composites, Solid State Ionics 76: 23-28 (1995). These patents and this technical journal article are all incorporated herein by reference in their entireties. In order to develop a membrane suitable for use in pressure driven oxygen separation, an electronic conductivity characteristic has to be added to pure ionic conductors, thereby creating multiphase mixed conductors. This is typically accomplished by adding an electronically-conductive phase, such as Pt or Pd, to the ionic conductor in volume percentages above the percolation limit (typically greater than 30%) to obtain a continuous electronically conducting phase that exists as a continuous interpenetrating network with the oxide ion conducting phase.
In contrast to multi-phase mixed conductors, true mixed conductors, which are exemplified by perovskites such as La.sub.0.2 Sr.sub.0.8 CoO.sub.x, La.sub.0.2 FeO.sub.x, La.sub.0.2 Sr.sub.0.8 Fe.sub.0.8 Co.sub.0.1 Cr.sub.0.1 O.sub.x and others, are materials that possess intrinsic conductivity for both electrons and ions in a single phase. Some of these materials possess some of the highest oxygen ion conductivities known, as well as rapid surface exchange kinetics. U.S. Pat. No. 5,702,999 (Mazanec et al.) and U.S. Pat. No. 5,712,220 (Carolan, et al.) disclose mixed oxide perovskites of this type that are useful for oxygen separation. However, while there is great potential for these materials in gas separation applications, there are some drawbacks in their use.
A common problem among most ceramic mixed conductors, including perovskites, is their fragility and low mechanical strength in tension, which makes it difficult to fabricate large elements, such as tubes, and deploy them in commercial systems requiring high reliability. These problems have been recognized and reported in technical journal publications, such as, for instance, Yamamoto et al. in Perovskite-Type Oxides as Oxygen Electrodes for High Temperature Oxide Fuel Cells, Solid State Ionics 22: 241-46 (1987); and B. Fu et al. in (Y.sub.1-x Ca.sub.x) FeO.sub.3 : A Potential Cathode Material for Solid Oxide Fuel Cells, Proc. 3rd Intl. Symp. on Solid Oxide Fuel Cells, S. C. Singhal, Ed., The Electrochem. Soc. Vol. 93-4: 276-282 (1993).
U.S. Pat. No. 5,911,860 discloses dual phase solid electrolyte ion transport materials comprised of a mixed conductor such as perovskite and a second phase such as Ag, Pd or an Ag/Pd alloy. This patent discloses that the introduction of a metallic second phase to a ceramic mixed or pure ion conductor such as perovskite prevents microcracking during fabrication of the membrane, and enhances the mechanical properties and/or surface exchange rates, as compared to those provided by a mixed conductor phase alone.
The introduction of a metallic second phase into ceramic mixed conductors is thus desirable for solid electrolyte ion transport membrane manufacture, not only for ceramic conductors, where the metallic phase is needed to achieve electronic conductivity, but also for true mixed conductors such as perovskites, where the metallic phase enhances mechanical properties and/or catalytic performance, as well as possibly enhancing the desired electronic conductivity. The most common technique disclosed in the prior art for introducing a metallic second phase into a solid electrolyte ion transport membrane is powder mixing. Illustrative of powder mixing techniques are the following patents:
(A) U.S. Pat. No. 5,306,411 (Mazanec et al.) discloses a typical powder mixing process to fabricate solid electrolyte ion transport membranes comprising gas impervious, multi-phase mixtures of an electronically-conductive material and an ion-conductive material and/or gas impervious, single phase mixed metal oxides of a perovskite structure. A mixture of La (C.sub.2 H.sub.3 O.sub.2).sub.3.1.5H.sub.2 O, Sr(C.sub.2 H.sub.3 O.sub.2).sub.2 and Co.sub.3 O.sub.4 was placed into a polyethylene jar mill, together with ZrO.sub.2 media and acetone, and rolled for 70 hours. The resulting slurry was decanted and vacuum distilled at room temperature until dry. The solids were then calcined in air in an evaporating dish for 12 hours at 900.degree. C. and 6 hours at 1100.degree. C. PA1 (B) U.S. Pat. No. 5,712,220 (Carolan et al.), discloses a membrane containing a dense multicomponent metallic oxide layer formed from La.sub.0.2 Ba.sub.0.8 Co.sub.0.62 Fe.sub.0.21 O.sub.3-z. This composition was prepared by a powder preparation technique wherein various applicable weighed quantities of La.sub.2 O.sub.3, BaCo.sub.3, CoO, Fe.sub.2 O.sub.3 and CuO were mixed and ball milled for 12 hours. The mixture was then fired in air to 1000.degree. C. for 24 hours followed by cooling to room temperature. The mixture was then ground by ball milling, remixed and refired. The resulting perovskite powder was milled in air to about 1-5 micron particle sizes and combined with a plasticizer, binder and toluene solvent to form a slip, suitable for tape casting. PA1 (C) U.S. Pat. No. 5,624,542 (Shen et al.) is primarily concerned with improving the mechanical strength of an ion conducting dense membrane by inclusion of a second metallic phase within the matrix described therein. The patent's independent claims disclose continuous electronic conductivity for a range of volume percent of the electronic conducting metal phase within the range of between 10 and 50 percent. However, the lower limit of 10 percent appears to contradict the discussion in the specification (column 6, lines 1-25) of this patent, which reports findings of a lower limit of 20 to 35 volume percent and recommends lower limits of 1 to 5 percent above that value. These disclosures are not believed to suggest or describe the present inventors' preferred limit of volume percentages of the electronic conducting phase within the matrix of less than 20 percent. Also the Shen patent does not address methodology for producing porous matrices composed of two phases of ion conduction oxides and metals. Indeed, the manufacturing method disclosed by the Shen patent involves the production of a mixed ionic-electronic conducting ceramic/metal composite by ball milling, including the steps of mixing and grinding ceramic components with a metal powder or metal oxide, followed by forming and sintering to provide the desired membrane. Grinding of the metal and ceramic components in accordance with the '542 patent is said to produce a particle size for the ball-milled metal and ceramic components of from about 0.5 micron to about 1 micron.
Other techniques for adding second phase metallic materials to solid electrolyte ion transport membranes are also known. For example, U.S. Pat. No. 5,306,411 (Mazanec et al.) discloses a technique in which the ceramic precursor components are added to deionized water and the solution is spray-dried to produce small droplets having a diameter of about 20-50 microns. The droplets are then dehydrated with preheated dry air, resulting in a powder having an average particle size of approximately 5 microns.
U.S. Pat. No. 5,624,542 (Shen et al.) discloses generally, in column 6, lines 45-50 thereof, that mixed ionic-electronic conducting ceramic/metal composites can also be formed by chemical vapor deposition, electrochemical vapor deposition, dip-coating, and sol-gel processing. However, these methods differ in their result from the powder mixing and spray drying techniques described above. Because they are designed to be applied after the formation of a first phase membrane, these methods are more suited for the preparation of multi-layer separation membranes than composite mixed-conductor membranes. Thus, these prior art coating techniques are not suited for introducing a metal into solid electrolyte ion transport precursor materials prior to the formation of the solid electrolyte ion transport membrane.
Multi-layer separation membranes are known in the art. For example, Yasutake Teraoka et al. reported solid state gas separation membranes formed by depositing a dense mixed conducting oxide layer onto a porous mixed conducting support in Jour. Ceram. Soc. Japan. International Ed., Vol.97, No.4, pp.458-462 and No. 5, pp.523-529 (1989). The relatively thick porous mixed conducting support provides mechanical stability for the thin, relatively fragile dense mixed conducting layers. The article does not discuss two phase membrane materials. Other exemplary multi-layer ceramic membranes are disclosed in U.S. Pat. No. 4,791,079 (Hazbun); U.S. Pat. No. 5,240,480 (Thorogood et al.); U.S. Pat. No. 5,494,700 (Anderson et al.); and U.S. Pat. No. 5,342,431 (Anderson).
The Anderson et al. ('700) patent disclose a method for preparing a membrane substrate coated with a dense crack-free metal oxide film made by dissolving metal ions in a polymerizable organic solvent, such as ethylene glycol. Generally the method of the '700 patent comprises: (1) preparing a starting solution containing cations of the desired oxide's metal constituents dissolved in an aqueous mixture of the polymerizable organic solvent; (2) heating the starting solution to form a polymeric precursor; (3) depositing a thin film of the polymeric precursor onto a substrate using a conventional spin-coating technique; and (4) calcining the deposited precursor film to convert it into a polycrystalline metal oxide film.
The Anderson ('431) patent discloses a method for incorporating a metal oxide film onto a ceramic membrane comprising the steps of (a) passing a dilute colloidal suspension ("sol") of metal oxide particles suspended in water or alcohol by one side of a porous support, (b) converting the sol into a gel by removing the solvent, (c) drying the gel to form a "xerogel," and (d) sintering the xerogel to create a porous metal oxide ceramic membrane that is said to be useful in ultrafiltration, reverse osmosis, or gas separation.
In summary, the introduction of a metallic second phase into solid electrolyte ion transport membranes is a useful step in the fabrication of mixed ionic-electronic conducting ceramic composites, and creates materials with great potential for gas separation and solid oxide fuel cell electrodes. However, the techniques heretofore taught in the prior art for introducing a metallic second phase pose several difficulties for commercial utilization of this technology.
For instance, the existing techniques for introducing a metallic second phase into solid electrolyte ion transport membranes often require large quantities of the second phase metallic material, which increases costs and can lead to lower ionic conductivity of the mixture. In a simple dual phase mixed conductor system comprised of an oxygen ion conductive material and an electronically-conductive material, the percolation theory is usually used to predict the volume fraction of the second (metallic) phase required to achieve electronic conductivity in a mixed conductor system. The minimum value of the volume fraction required to achieve a continuous second phase typically falls in the range of about 30%, although this value can vary markedly, depending upon the relative sizing of the individual components.
Prior technical literature discloses that the metallic second phase usually constitutes more than 40% of the volume of the composite. This amount is typically necessary to ensure that the conducting phase is above the percolation limit in order to obtain a composite electronic/ionic mixed conductor. For example, a technical journal article Microstructural Development, Electrical Properties and Oxygen Permeation of Zirconia-Palladium Composites, Solid State Ionics 76: 23-28 (1995), C. S. Chen et al., reported that a percolative Yttria-stabilized cubic zirconia (YSZ)--palladium dual phase composite, containing 40% Pd by volume, showed a much larger oxygen permeability than that of a non-percolative composite containing 30% Pd by volume indicating a percolation limit between 30 and 40%. The high cost of a compatible second phase (e.g. Pd, Pt), coupled with the high volume required by the prior art techniques, makes it difficult to commercialize these solid electrolyte ion transport membranes.
Also, since the second phase is a pure electronic conductor, any excessive use of second phase material, which is typical of the prior art techniques, results in a reduction of the overall ionic conductivity of the composite, a clearly undesirable result for high performance in oxygen transport.
In the case of true mixed conductors, such as perovskites, to which a metallic second phase may be added to enhance mechanical properties and/or catalytic efficiency (see U.S. patent Ser. No. 08/775,683), conventional techniques for introducing the second phase may reduce the benefits derived from their use. In the prior art, dual phase solid electrolyte ion transport powders of these materials were usually prepared by mixing various weight ratios of second phase alloys and solid electrolyte ion transport powders using a conventional powder mixing process. However, during the conventional powder mixing process a non-uniform dispersion of the second phase can result in lower mechanical strength of the ceramic composite due to the lack of homogeneity of the mixed material.
There is need, therefore, for a new method for incorporating a metal or metal oxide into an ionic or mixed ionic/electronic ceramic membrane prior to fabricating the membrane in order to achieve a reduction in the amount of material required for the second phase and to attain a uniform deposition of the metal or metal oxide within the ceramic membrane substrate, thereby enhancing the mechanical properties and/or the overall transport efficiency of the membrane. There is also a need for the resulting improved membrane itself.